Exhaust gases from diesel engines contain particulate matter (PM) based on carbonaceous soot and SOFs (soluble organic fractions) of high-boiling-point hydrocarbons, and the release of PM into the atmosphere is likely to exert adverse effects on humans and environment. Exhaust pipes connected to diesel engines are thus conventionally provided with ceramic honeycomb filters (simply called “honeycomb filters” below) for capturing PM.
FIGS. 1(a) and 1(b) show one example of honeycomb filters for capturing PM to clean the exhaust gas. A honeycomb filter 10 comprises a ceramic honeycomb structure having porous cell walls 2 for constituting large numbers of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 and a peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c for sealing the exhaust-gas-inlet-side end surface 8 and exhaust-gas-outlet-side end surface 9 of the outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 alternately in a checkerboard pattern. The honeycomb filter is stationarily disposed in a metal container (not shown), with its peripheral wall 1 gripped by holding members (not shown) constituted by a metal mesh, a ceramic mat, etc.
An exhaust gas is cleaned by the honeycomb filter 10 as follows. As shown by dotted lines, the exhaust gas flows into the outlet-side-sealed flow paths 3 open on the exhaust-gas-inlet-side end surface 8, and while passing through the cell walls 2, specifically while passing through pores communicating on and in the cell walls 2, PM contained in the exhaust gas is captured. The cleaned exhaust gas is discharged from the inlet-side-sealed flow paths 4 open on the exhaust-gas-outlet-side end surface 9 to the air.
The honeycomb filter is required to have high PM-capturing efficiency and small pressure loss. To meet these requirements, the cell walls have proper porosity, and most cell walls have porosity of about 50-65%. Presently available honeycomb structures for constituting the honeycomb filters are mainly made of cordierite.
The honeycomb filter should have such heat resistance that it is not destroyed or melted by high heat generated when the captured PM is burnt. To meet such requirement, aluminum-titanate-based honeycomb filters having similar thermal expansion coefficients to and higher melting points than those of cordierite have been finding practical applications in place of cordierite-based honeycomb filters.
The low thermal expansion of aluminum titanate appears to be due to microcracks existing in its sintered bodies. More microcracks provide smaller thermal expansion coefficients, though providing sintered bodies with lower strength. Further, because of relatively large sintering shrinkage, aluminum titanate is likely to suffer sintering cracking.
JP 6-40766 A discloses an aluminum-titanate-based ceramic comprising aluminum titanate as a matrix and mullite or a mullite cordierite composite, which has high strength and a low thermal expansion. Its Examples show ceramics having thermal expansion coefficients of about 1.9-6.1×10−6/° C.
Although such thermal expansion coefficient of this ceramic may be sufficient for insulators for engines, it is insufficient for large honeycomb structures having outer diameters of 100 mm or more and lengths of 150 mm or more for PM-removing filters for diesel engines.
JP 2005-534597 A discloses an aluminum-titanate-based ceramic having a composition represented by u(Al2O3—TiO2)+v(R)+w(3Al2O3-2SiO2)+x(Al2O3)+y(SiO2)+z(1.1SrO-1.5Al2O3-13.6SiO2—TiO2)+a(Fe2O3—TiO2)+b(MgO-2TiO2), wherein R represents SrO—Al2O3-2SiO2 or 11.2SrO-10.9Al2O3-24.1SiO2—TiO2, u+v+w+x+y+z+a+b=1, 0.5<u≦0.95, 0.01<v≦0.5, 0.01<w≦0.5, 0<x≦0.5, 0<y≦0.1, 0<z≦0.5, 0<a≦0.3, and 0<b≦0.3, which has a low thermal expansion coefficient, high heat shock resistance and high gas permeability, and its Examples show ceramics having thermal expansion coefficients of 0.9×10−7/° C. to 11×10−7/° C. The ceramics of JP 2005-534597 A are substantially composed of titanate aluminum, 3Al2O3-2SiO2 (mullite), and SrO—Al2O3-2SiO2, which are obtained by sintering a plasticized mixture comprising inorganic materials comprising silica, alumina, strontium, titania and iron oxide, organic molding aids comprising a plasticizer, a lubricant and a binder, and water. However, because titanate aluminum, 3Al2O3-2SiO2 (mullite) and SrO—Al2O3-2SiO2 are synthesized during a sintering process in the production method, the ceramic suffers large sintering shrinkage, undergoing cracking in the production process. Particularly when large honeycomb structures of 100 mm or more in outer diameter and of 150 mm or more in length for PM-removing filters for diesel engines are produced, cracking due to the sintering shrinkage is a serious problem.
JP 11-114336 A discloses an exhaust gas filter formed by a porous ceramic containing amorphous particles of Al2O3 and SiO2 between and on aluminum titanate crystal particles, describing that this ceramic has a low sintering shrinkage ratio and good dimensional accuracy. JP 11-114336 A describes that in the sintering of a ceramic material comprising 100 parts by weight of aluminum titanate and 5-20 parts by weight of clay particles, clay particles become amorphous between and on coarse aluminum titanate particles, so that the coarse particles are bonded strongly. However, the use of clay as a starting material makes it difficult to sufficiently lower the sintering shrinkage ratio in the sintering process by the influence of a liquid phase generated from 5-20 parts by weight of clay particles. For instance, this method fails to obtain a sintering shrinkage ratio of less than 10%. Although clay may be effective to aluminum titanate having porosity of 40% or less to some extent, it does not provide honeycomb structures having porosity of 45% or more with sufficiently satisfactory strength and thermal expansion coefficient. Particularly when the porous ceramic is used for large honeycomb structures having outer diameters of 100 mm or more and lengths of 150 mm or more for PM-removing filters for diesel engines, the honeycomb structures do not have sufficiently satisfactory sintering shrinkage ratio, strength and thermal expansion coefficient.